Two-dimensional circuits have an open electrode structure which gives rise to fringing fields above the surface of the circuit. External electro-optic (e-o) probing techniques exploit the fringing fields to electro-optically sample fast electronic and opto-electronic devices and circuits. An e-o crystal probe placed into the fringing fields changes the crystal birefringence of the probe, which can then be measured optically by means of a pulsed light beam which is passed through the e-o crystal and is reflected from the circuit. Such external probes can be applied to almost any type of circuit, because the interaction is based on the field effect. Since no charge is removed from the circuit, the probe does not need to make an electrical contact to the circuit. Some of the electro-optic techniques and apparatus are disclosed in articles by J. A. Valdmanis and G. Mourou, "Electro-Optic Sampling: Testing Picosecond Electronics, Part 1, Principles and Embodiments", Laser Focus/Electro-Optics, February 1986, pages 84-96, and "Electro-Optic Sampling: Testing Picosecond Electronics, Part 2, Applications", Laser Focus/Electro-Optics, March 1986, pages 96-106; J. F. Whitaker et al., External Electro-Optic Integrated Circuit Probing, Elsevier Science Publisher, B.V. 1990, pages 369-379, and in U.S. Pat. No. 4,891,580 issued Jan. 2, 1990 to Janis A. Valdmanis.
In FIGS. 6 and 7 is shown a typical prior-art arrangement, 10, being used for optical sampling of an exemplary circuit having a plurality of conductors, 12, on a surface of a semiconductor substrate, 13. Illustratively, the substrate and conductors are part of an integrated circuit. An electro-optic (e-o) crystalline probe 15, is mounted on a transparent support, 16, forming a supported probe, 14. An end face of probe 15 is positioned over or on the conductor whose voltage waveform is to be sampled. Sampling is performed optically by short-duration pulses of a polarized light beam 17 which is directed through support 16 into probe 14. Beam 17 is typically reflected from the conductor itself, but it may also be reflected by an optional reflector 18 (shown in dashed line in FIG. 7) disposed on the end face of the probe. The latter may be desirable in cases where the circuit is sensitive to light.
In order to effect voltage measurements, the e-o crystal of probe 15 comprises a material which exhibits a longitudinal e-o effect; that is, a field-induced birefringence in response only to electric field components parallel to the direction of beam 17. Since the beam is directed perpendicularly to the surface of the conductor, only field components perpendicular to that surface induce the desired birefringence. The material itself need not have an inherent birefringence.
Sampling pulses are generated by a pulsed laser 19, such as the well-known balanced colliding pulse mode-locked (CPM) dye laser or a passively mode-locked titanium doped sapphire laser, and are directed into the sampling apparatus via a microscope objective 21. Probe 14 is mounted optically (not physically) between crossed polarizers 22 and 23 and below a dichroic beam splitter 24 in such a way as to facilitate viewing of the end face of the probe from above via a microscope system 25. Incoherent white light is injected from an illumination source 26 through beam splitter 24 to illuminate the conductor below the probe. In this way, both the conductor and the sampling beam spot can be seen together. Quartz compensating plates 27 are included between polarizers 22 and 23 to operate the e-o crystal at a "zero-order" quarter wave point. Polarizer 23, which is a polarizing analyzer, is used to separate orthogonal polarizations and direct the output to dual differential light detectors 28 and 29. The detector output is then fed to a lock-in amplifier and signal averager, 30. The probe is then brought down to any suitable point on or above substrate 13 for sampling the voltage waveform of a selected conductor.
A voltage waveform may be generated on the conductor by coupling a test signal thereto (or to the circuit of which it is a pan) either electrically via a suitable (e.g. high-speed) electrical connection 31 or optically via optical delay line 32 coupled to a photodetector (not shown) on substrate 13. In either case, well-known sampling techniques are utilized so that the optical pulses and the waveform being measured are synchronized, and the optical pulses are scanned across the waveform to be sampled.
In operation, polarized beam 17 is illustratively used to sense change in the birefringence induced in the e-o crystal. When the conductor being measured has zero voltage on it, then the reflected beam (immediately before polarizer 23) has orthogonal polarization components which are 90 degrees out of phase with one another. These components are separated by the polarizing analyzer (polarizer 23) and generate equal signals (balanced outputs) from detectors 28 and 29. However, when the voltage on the conductor is not zero, the birefringence induced in the crystal is changed, and an additional phase shift is produced between the two orthogonal components; i.e., the reflected beam is elliptically polarized, resulting in an imbalanced output from detectors 28 and 29 and a signal from lock-in amplifier/averager 30 which is proportional to the voltage on the conductor.
Typically, the prior-art crystal probe 15 comprises a relatively thick slice of LiTaO.sub.3, e.g., about 100 .mu.m thick, and the support 16 is a transparent material such as fused silica rod bonded to the probe. The probe and a portion of the rod adjacent to the LiTaO.sub.3 slice are polished as a four-sided pyramid having a half angle of about 30 degrees with the end face of the probe being 200 .mu.m or less on a side. Deviations from 30.degree. result in reduced facet reflectivities. The optic axis of the LiTaO.sub.3 is essentially perpendicular to the end face of the probe. If needed, an optional high reflecting (HR) coating 18 (FIG. 6) may be evaporated onto the end face of the probe so that the sampling beam can be reflected directly back to the optical system without reflecting of the side facets. Microscope objective 21 is used both to focus the sampling beam onto the end face of the probe and to recollimate the reflected beam.
Conductors 12 on substrate 13 (i.e., the circuit) are energized via either a waveform synthesizer (not shown) coupled to high-speed connection 31 and synchronized with the laser or vice-versa by frequency locking the laser to a master RF source or via an electrical signal from a photodetector (not shown) illuminated by the laser pulses from delay line 32. Electrical connections to the circuit may be made by conventional probes, a probe card, or wire bonding. Relative delay between trigger and sampling times is introduced by the motor driven optical delay line 32, although purely electronic means, in some cases, can also be utilized. Utilizing apparatus of this type with optical pulses of about 100 fs duration, it is expected that measurement bandwidths in excess of 1 THz (1000 GHz) can be attained.
The measured electrical rise times are often longer than the optical pulses. This difference is due in part to the distance the electrical signal must travel from the excitation site to the sampling site, and in part to the distance the optical sampling pulse must travel through the LiTaO.sub.3 sampling crystal. As the electrical pulse travels along the transmission line it is dispersed, resulting in preferential attenuation and radiation of the high frequency components. As the optical pulse travels in the LiTaO.sub.3 crystal it propagates with the electrical pulse, but at a different velocity. This results in velocity walk-off, causing the detected signal to appear longer. This walk-off effect becomes enhanced as the crystal thickness is increased.